(1) Field of the Invention
The present invention relates to an emulsion-polymerization process and to soluble, electrically-conductive polyaniline salts, and more particularly to an emulsion-polymerization process for producing a polyaniline salt of a hydrophobic organic acid wherein the polyaniline salt is soluble in organic solvents.
(2) Description of the Prior Art
Polyaniline is a polymer recognized as being chemically stable as well as electrically conductive in the protonated form. Nevertheless, use of polyaniline has been limited because it has been considered intractable or unprocessible.
Synthesis of polyaniline is commonly performed by the method of chemical oxidative polymerization based upon an aqueous solution polymerization system. (see Cao et al., Polymer, 30:2305, 1989, which is incorporated by reference). This method involves combining water, a protonic acid, aniline and an oxidizing agent and allowing the mixture to react while maintaining the reaction mixture at constant temperature. After a period of several hours, the precipitated polyaniline is separated from the reaction mixture and washed. Material synthesized by this approach is insoluble in organic solvents and predominantly amorphous. (Annis et al., Synth. Met. 22:191 et seq., 1986 which is incorporated by reference).
A second step after polymerization has been utilized to improve processibility consisting of doping the polyaniline to the conducting form with acids and dissolving in organic solvents. Tzou and Gregory (Synth. Met., 53:365-377, 1993 which is incorporated by reference) used this approach and reported that polyaniline salts containing carboxyl and amino substituents were soluble in the polar solvents, N-methyl-2-pyrrolidinone and dimethylsulfoxide. In contrast, in the same solvents the polyaniline salts of dodecylbenzenesulfonic acid, 1,5-naphthalenedisulfonic acid and .rho.-toluenesulfonic acid were insoluble or were unstable and precipitated.
Cao et al. reported on use of this method to prepare polyaniline salts of a number of protonic acids including 4-dodecylbenzene sulfonic acid and dinonylnaphthalene sulfonic acid. (Cao et al., Synthetic Metals 48:91-97, 1993; Cao et al., U.S. Pat. No. 5,232,631, 1993 both of which are incorporated by reference). The polyaniline salts were reported to be soluble in nonpolar solvents such as xylene, however, the solubility appeared to be very low. For example the maximal solubility of the dodecylbenzenesulfonic acid salt of polyaniline was less than 0.5%.
In one variation of this process, the polyaniline was doped with one material predominantly at core of the particles and with a second material predominantly at the surface. The surface dopants were identified to be a number of possible protonic acids including dinonylnaphthalenesulfonic acid.(Shacklette et al., U.S. Pat. No. 5,281,363 which is incorporated by reference). It is disclosed that use of the surface dopant is to achieve increased compatibility between the conductive polymer particles dispersed in a matrix polymer of lower surface energy and lesser polar character.
Emulsion polymerization processes for preparing a polyaniline salt of a protonic acid have been reported. (Cao et al., U.S. Pat. No. 5,232,631, Example 6B, 1993; Cao and Jan-Erik, W094/03528, 1994 I; Cao and Jan-Erik, U.S. Pat. No. 5,324,453, 1994 II; see also, Osterholm et al., P. Synth Met 55:1034-9, 1993 all of which are incorporated by reference). In these disclosures aniline, a protonic acid, and an oxidant were combined with a mixture of a polar liquid, typically water, and a nonpolar or weakly polar liquid, e.g. xylene, chloroform, toluene, decahydronaphthalene and 1,2,4-trichlorobenzene, all of which are either sparingly .soluble or insoluble in water. There was, however, no disclosure in the examples of the use of any non-polar or weakly-polar liquid having a solubility in water greater than one part in 200 parts water. Furthermore, because this group used organic solvents that were insoluble in water, the protonic acid was required to have substantial emulsifying properties. Moreover, the solvents used by this group are considered hazardous materials such that the polyaniline formed could not be used in coatings, finishes, paints, inks and the like, or in blends with other components without first separating the polyaniline from the solvent.